The spectral efficiency of communication networks can be improved by aggressively reusing scarce radio resources in neighboring cells. As a result, inter-cell interference has become a main source of signal disturbance, limiting not only the service quality of the cell-edge users but also the overall system throughput. Coordinated multi-point (CoMP) transmission and/or reception is becoming more widely adopted due to its promise in effectively mitigating inter-cell interference. The basic idea behind CoMP in downlink is to connect multiple base stations from several adjacent cells to form a “super-cell”, or a so-called CoMP cell, such that transmissions to multiple mobile stations within each CoMP cell can be coordinated to reduce or even avoid mutual interference among the mobile stations.
Zero-forcing (ZF) linear precoding has been employed for combating inter-cell interference in a CoMP downlink. ZF linear precoding allows simultaneous transmission over the same frequency band to multiple users without creating any mutual interference within a CoMP cell by sending the signal of each user in a direction that is orthogonal to the channels of other users. Although ZF precoding has low computational complexity and performs well when the network has low load, at high load, however, the transmitter can run out of orthogonal dimensions, causing most of the users to transmit and/or receive at unfavorable directions which in turn leads to low signal-to-noise-plus-interference-ratio and thus limits the overall system throughput.
MMSE (minimum mean-squared error) linear multi-user precoding has been proposed as an alternative to ZF precoding. In principle, an MMSE precoder can advantageously weight down the importance of those users with weak channel responses and thus open-up the available signal dimension at high load. However, unlike their receiver counterpart, multi-user MMSE precoders do not have closed-form expressions in general. Mainly, the choice of precoders that minimizes the MSE (mean-squared error) depends on the choice of the corresponding receiver, and vice versa, leading to a set of optimality conditions that are no longer linear as in the case of MMSE receivers.
Since multi-user MMSE precoders do not have closed-form expressions, most of the algorithms available for computing precoding weights are based on numerical optimization routines which typically require proper choices of some associated numerical parameters to guarantee convergence. Moreover, most MMSE precoding techniques assume the mobile stations have only one receive antenna. However, in next-generation cellular communication systems, most mobile stations will likely have more than one receive antenna. In addition, most MMSE preceding techniques assume that the channel response between each mobile station UE and each base station is known perfectly at the transmitter. This assumption is unrealistic in practice due to the feedback delay and/or the limited amount of pilot sequence information available in typical cellular systems.